Electromechanical systems, particularly the type that handle large libraries of stored data such as a disk library, can cause physical destruction of the data recorded on the recording media if a mechanical and/or electrical failure is not detected and all machine movement halted as a result of that detection. Various commercially available sensors are employed utilizing various techniques such as optical or inductive proximity for sensing mechanical position. These devices require additional wiring and bracketry which can adversely affect cost and reliability of the system. In addition, systems which utilize switch contacts tend to degrade over time and become unreliable making system protection illusionary. Solenoids provide an effective means for linking electronic signals to mechanical motion. In addition, there is a particular property that they also exhibit that can provide a means for detecting position. That is, the property of inductance of a coil, in this case, the coil of the solenoid which changes depending on the position of the iron core plunger with respect to the solenoid body. By sensing this inductance, it is possible to detect the occurrence of a mechanical event through the linkages associated with the plunger.
A patent of interest for its teaching is U.S. Pat. No. 4,387,306 entitled "Vital Contact Checking Circuit" by H. C. Sibley. The invention of that patent is concerned with railroad switch safety in that a determination of the position of a railroad switch whether open or closed is extremely important to the well-being of the trains running over the switch. To provide this detection, a transformer primary winding is used as a component of a serially tuned resonant circuit by the addition of a capacitor component and a signal source. The secondary winding passes on the transformed signal to an open and/or closed indicator circuit. A third winding is provided on the transformer with terminals that are connected to a contact switch, which switch in turn is controlled by the actual positioning of the railroad switching element.
In operation, when the third winding is shorted by the contacts of the switch, a detuning occurs in the series resonant frequency response of the primary winding and the capacitor to cause a diminished amplitude in the signal that is transformed to the secondary winding. This difference in signal amplitude is indicative of the positioning of the railroad switch contact.